What You Need to Know About Volcanoes

Most of us are unlikely to ever be directly affected by a volcano, but they’re still absolutely fascinating. They come in far more varieties than most people think, so let’s take a closer look at the different kinds of volcanoes and eruptions.

Most volcanoes around the world are located along plate boundaries, though sometimes they also occur in the middle of tectonic plates. Where plates are pulling apart, such as at the Mid-Atlantic Ridge, lava wells up to create new seafloor and occasionally, volcanic islands (such as Iceland). At convergent faults, one plate melts as it’s subducted under the other, causing plumes of hot material to well up. Examples include the volcanoes of the Pacific Northwest, such as Mount St. Helens and Mount Hood. Other volcanoes occur atop “hot spots” in the middle of plates; the most famous example being the Hawaiian Island chain. As plates move over these spots, new volcanoes form and the old ones die off. While geologists once believed that such hot spots were the result of plumes of molten material originating deep within the mantle, new research shows that the superheated material may be confined to “pools” within the upper mantle (though pressures at the depths at which they are found prevent the rock from actually melting).

There are several basic categories people use when discussing volcanoes, though since there are no hard and fast guidelines to distinguish them, they aren’t of particular use to scientists.

• Extinct volcanoes no longer have a source of lava and (theoretically) pose no danger of future eruptions. Sometimes volcanoes go extinct because they’ve moved away from a hot spot, while others may be found along ancient plate boundaries that are no longer active. However, it may be difficult to tell if a volcano is truly extinct or merely dormant since some kinds can go tens, if not hundreds, of thousands of years between eruptions. 
• Dormant volcanoes are trickier to define. In general, we use this description for volcanoes that haven’t erupted in several thousand years (more or less, within recorded history). Just because a volcano is considered to be dormant doesn’t mean it can’t still become active; many we thought were dormant have erupted in recent years. 
• Active volcanoes are those with a geologically recent history of erupting. Depending on who you ask, this can mean within 10,000 years or if there’s a written mention of an eruption (though this is problematic since written records don’t exist for the same time frames in different regions, and even today we can only detect eruptions at some remote volcanoes via satellite imagery, meaning relatively recent eruptions that predate satellites might have gone unrecorded). However, just because a volcano is considered to be active doesn’t mean that it’s necessarily likely to erupt any time soon, if ever again.
• The most obvious category is erupting volcanoes. Eruptions can range from the full-out explosive events that most people picture, to a fairly gentle seep of lava and/or ash. A volcano can also be categorized as “likely to erupt” if there has been a sudden increase in earthquake activity or gas releases, though it’s currently impossible to say precisely when or if a particular volcano will erupt.

The severity of eruptions is measured on the Volcanic Explosivity Index (VEI), which is a (mostly) logarithmic scale of the amount of ejecta produced by an eruption, which generally correlates to how long the eruption lasts. The lowest level is VEI 0, which consists of a continuous eruption with a plume of less than 100 m in altitude and very little total output; the most obvious example is the eruption at Kilauea that’s been ongoing since 1983. At the other end of the scale at VEI 8 are supervolcanoes, which can eject more than 10,000 cubic kilometers of material and have a plume more than 50 km (31 mi) high. To give you an idea of just how strong that is, Mount St. Helens only rated a VEI 5 while the famous 1883 eruption of Krakatoa was a VEI 6. Fortunately, these are relatively rare; the last such eruption was about 26,500 years ago at Taupo Volcano in New Zealand.

The scale roughly corresponds to the different common names for varying magnitudes of magmatic eruptions. The least powerful are Hawaiian eruptions, which tend to be long-lasting but not explosive. You can walk right up the the lava flows at Kilauea in relative safety (see right), even to the cone at Pu’u O’o if you’re feeling particularly adventurous. (Despite the name, they do occur outside the Hawaiian volcanoes.) Strombolian eruptions are named after Stromboli, a volcano off the coast of Italy that’s been erupting for centuries; these eruptions happen when gas bubbles pop and throw lava into the air. In Vulcanian eruptions (after Vulcano, another island near Stromboli), magma within the volcano is too viscous for gas to escape, resulting in a series of short-lived but moderately powerful eruptions as pressure repeatedly builds up until the cap blows off. Peléan eruptions are much more dangerous because they blow out massive amounts of material, resulting in landslides, ash falls, and pyroclastic flows (fast-moving clouds of superheated gas and rocks). The name arises from the 1902 eruption of Mount Pelée on the Caribbean island Martinique, which destroyed an entire city and killed approximately 30,000 people. The most powerful magmatic eruptions are Plinian or Vesuvian eruptions, after Pliny the Younger, who observed and described the eruption of Vesuvius that destroyed Pompeii in AD 79. These occur when a narrow magma conduit within the volcano creates a large eruptive column, which causes pumice falls, ash falls, and pycroclastic flows when the column collapses. When these eruptions are particularly strong, they may be called Ultra-Plinian.

There are also a few other types of eruptions that don’t fit our typical idea of volcanoes. Phreatomagmatic eruptions are those in which the vent is underwater and the magma interacts with seawater (or more rarely, where the magma interacts with an underground aquifer to produce the same sort of explosion). Submarine volcanoes occur in deep water and may form seamounts and eventually, islands. Surtseyan eruptions are in shallow water, where the magma and seawater interact to actually shoot steam and ash out of the water, sometimes forming islands such as Iceland’s Surtsey, which formed in 1963. Subglacial eruptions happen when volcanoes start to erupt underneath an ice sheet or glacier, eventually melting the ice above and forming a lake, which then boils away. Phreatic eruptions happen when steam buildup causes existing material within the volcano to explode outward, but without any magma coming to the surface.

There is no single alert system for volcanic eruptions. The US Geological Survey uses four levels to describe a volcano’s activity level:

• Normal (green) – The volcano is showing normal non-eruptive activity.
• Advisory (yellow) – The volcano is showing slightly elevated activity levels.
• Watch (orange) – Activity has escalated, and there may be small eruptions that pose no danger.
• Warning (red) – An eruption is imminent or already underway.

Similar warning levels exist for air traffic; flights need to be grounded or rerouted if there’s a significant ash cloud since airplanes and ash do not mix well. If you find yourself in the path of a lava flow, lahar (volcanic mudslide), or pyroclastic flow, your best bet is to try to evacuate, though there are some precautions you can take.